Solenoid-operated devices are used in many different situations to convert electrical energy into linear motion. For example, a solenoid switch is a type of relay that uses an internal electromechanical solenoid to operate a switch in an electrical circuit. Solenoid valves are integrated devices containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve. Solenoid valves are used in fluid power systems, to control pneumatic or hydraulic cylinders, and in aircraft e.g. for fuel systems (fuel control, firewall shutoff, etc.) and environmental control systems (engine bleed air control, canopy seal inflation, coolant flow, etc.).
One application of hydraulic solenoid valves is to apply the brakes to the wheels of an aircraft. Another application of hydraulic solenoid valves is to apply the wing tip brakes which maintain symmetrical extension of the left and right flaps for each wing. The wing tip brakes lock the flaps and prevent further movement when a failure occurs in the aircraft actuation system. Typically the wing flaps of an aircraft are driven by a single motor with two shafts. If one shaft jams or breaks, then the wing tip brake operates to stop any movement of the other shaft and avoid the problem of unsynchronous flap extension. Once applied, the wing tip brake cannot be released in flight.
A particular issue facing wing tip brakes is that the hydraulic solenoid valve is rarely operated, only in a failure situation, but it must be primed to act quickly in case it is needed. Wing tip brakes are exposed to very harsh environmental conditions, with temperatures during flight down to −55° C. The operating times of hydraulic solenoid valves are adversely affected at low temperature due to increased fluid viscosity. Even beyond hydraulic systems, solenoid-operated devices may in general suffer a reduction in performance where drags and efficiencies are impacted by low temperature.
One way to ensure good performance in aircraft hydraulic systems is to heat the hydraulic fluid in response to low ambient temperatures. Separate electrical heating systems have been tried, but are not practical insofar as space, weight and efficiency are concerned. There remains a need for an efficient self-heating method in hydraulic solenoid valves and other solenoid-operated devices.